1. Field of the Invention
The present invention relates to a toner supply roller in a developing apparatus in a printer of electrophotographic system, electrostatic recording system and the like; and the developing apparatus. More particularly, the present invention pertains to a toner supply roller mounted on a developing apparatus which has a toner cartridge housing a toner as a developer, a toner supply roller and a developing roller, and which forms a toner image by supplying an electrostatic latent image on the surface of a photosensitive body with the toner; and the above-mentioned developing apparatus.
2. Description of the Related Arts
In recent years, there has widely been introduced an electroconductive roller system in a developing roller and toner supply roller that are employed in a developing apparatus which is arranged on a developing process in an electrophotographic apparatus such as an electronic copying machine, laser beam printer and facsimile machine. In the aforesaid system, the toner supply roller is rubbed against the developing roller so as to frictionally electrify the toner, and thus is required to possess a stable friction property (pressing force) with the developing roller and also an enhanced toner supply property to frictional portions and positions.
Such being the case, in the conventional toner supply roller of this type, there is generally employed a foamed elastic material wherein rubber or polyurethane is foamed.
However, as a result of investigation made by the present inventors on the performance of the conventional toner supply roller comprising the foamed elastic body, it has been proved that a defect due to unevenness in pitch or density sometimes occurs in a developed image.
In addition, the above-mentioned electroconductive roller system is characterized in that {circle around (1)} the product cost can be lowered because of non-use of an expensive magnet roller, {circle around (2)} a unary non-magnetic developer (toner) can be used, and {circle around (3)} the space can be curtailed. Such electroconductive developing roller and electroconductive toner supply roller are required to have a low electric resistance. 
In the electroconductive roller of this type, there has hitherto been generally used an electroconductive high molecular material such as rubber and polyurethane that are blended with carbon black or an electroconductivity imparting agent.
However, comparing a developing roller which is used always in contact with a drum composed of a photosensitive body with a toner supply roller which is used always in contact with a toner as a developer, the materials which are suitable for the respective objects are selected for use because of different characteristics to be taken into consideration for the other object in contact therewith, and also the materials which have each a low electric resistance are used for the purpose of manifesting electroconductivity. In spite of the above-mentioned consideration, it has been proved that a defect sometimes takes place in a developed image.
A general object of the invention, which has been made in the light of such circumstances, is to provide an toner supply roller that is composed of a foamed elastic body and is capable of affording an image free from such defect as unevenness in pitch or density, and at the same time, to provide a developing apparatus on which the toner supply roller is mounted.
Another object of the invention is to provide a developing apparatus which always forms an image free from any defect in the development by means of electroconductive roller system.
Still another objects of the invention will be obvious from the content of the specification hereinafter disclosed.
Investigations were made by the present inventors on the toner supply rollers which previously generated the aforestated defective image. The results are as follows:
(1) It has been found out that the hardness of a toner supply roller, when being unreasonably high, results in the occurrence of unevenness in pitch in an image due to variation in the rotational torque of a developing roller, whereas said hardness, when being unreasonably low, causes the occurrence of unevenness in density in an image due to the incapability of sufficiently supplying the developing roller with a toner. In particular, it has also been found that when an image having a high density such as a black solid image is printed immediately after the printing of an image having a low density such as a white solid image, there is generated unevenness in density in which the printed tip end density of the black solid image is higher than the printed rear end density thereof; in the case of white solid printing, because of a small amount of conveyed toner from a developing roller to a photosensitive body, the above-mentioned unevenness in density brings about a large amount of residual toner on the developing roller; thereby insufficiency in scraping of the toner with the toner supply roller is prone to occur, thus gradually increasing the amount of residual toner on the developing roller; and when the black solid printing is carried out subsequent to the white solid printing under such conditions, the aforestated unevenness in density is caused by that the image density in the printing during one to two revolutions of the developing roller is higher than the image density in the printing at the rear end portion.
As a result of further accumulated research, it has been found out that a variance in the hardness of a toner supply roller, if present, brings about partially different pressing force against the developing roller, and thus causes partially strong pressing force and partially weak pressing force between the toner supply roller and the developing roller, failing to assure stable friction property (pressing force); and as a result, the rotational torque of the developing roller varies, causing unevenness in pitch in an image, and/or the toner supply property varies, causing defective image such as unevenness in density in an image. It has further been found out that the toner scraping property with the toner supply roller is closely concerned with both the foamed cell diameter in a foamed elastic body and the hardness on and around the surface thereof as well as both the number of cells therein and the hardness on and around the surface thereof.
It has still further been found out that favorable images free from a defect can be formed by uniformly and properly setting the hardness of the toner supply roller expressed in terms of compression spring constant capable of expressing in high precision and accuracy, the partial hardness of a foamed elastic body, and that more favorable images free from a defect can be formed by properly and suitably setting the foamed cell diameter in a foamed elastic body, the number of cells therein as well as the hardness of the toner supply roller expressed in terms of compression spring constant.
(2) It has been found out that the pressing force of a toner supply roller against a developing roller, when being unreasonably strong, results in the occurrence of unevenness in pitch in an image due to variation in the rotational torque of a developing roller, whereas said pressing force thereof, when being unreasonably weak, leads to the occurrence of unevenness in density in an image due to the incapability of sufficiently supplying the developing roller with a toner.
As a result of further accumulated research, it has been found out that improper cutting amount of a developing roller into a toner supply roller in spite of a proper hardness of the toner supply roller brings about failure to assure proper pressing force, thus causing failure to assure stable frictional force; and as a result, there take place defective images such as unevenness in pitch and unevenness in density due to varied toner supply property.
It has still further been found out that favorable images free from a defect can be formed by properly setting the cutting amount of a developing roller into a toner supply roller and at the same time, by properly setting the pressing force of the toner supply roller against the developing roller.
(3) It has been found out that defective images take place in the case where the electric resistance of a toner supply roller is higher than that of a developing roller. As a result of further accumulated research, there were obtained such findings and information in that materials matching respective functions of these two electroconductive rollers can be selected, and favorable images free from a defect can be formed in the case where the materials for these rollers are selected so that the electric resistance of a toner supply roller is always lower than that of a developing roller even if the electric resistance of each roller varies accompanying the change in environment such as temperature and humidity.
The present invention has been accomplished on the basis of the foregoing findings and information.
That is to say, the present invention provides a toner supply roller that is composed of a foamed elastic body and is mounted on a developing apparatus which has a toner cartridge housing a toner as a developer, a toner supply roller and a developing roller, and which forms a toner image by supplying an electrostatic latent image on the surface of a photosensitive body with the toner, characterized in that said toner supply roller has a compression spring constant in the range of 0.25 to 5.0 N/mm; a developing apparatus on which the above-mentioned toner supply roller is mounted; and an image formation apparatus equipped with the aforesaid developing apparatus.
In addition, the present invention provides a developing apparatus which has a toner cartridge housing a toner as a developer, a toner supply roller and a developing roller, and which forms a toner image by supplying an electrostatic latent image on the surface of a photosensitive body with the toner, characterized in that the toner supply roller and the developing roller are each electroconductive, and the electric resistance of the toner supply roller is always lower than the electric resistance of the developing roller at an impressed voltage in the range of 100V to 500V; and an image formation apparatus equipped with the developing apparatus just described.